(a) Field of the Invention
The present invention relates generally to Yb-doped fiber lasers. More particularly, it relates to an optimum host glass for a Yb-doped fiber laser operating near the absorption band of an Er-doped fiber amplifier (EDFA).
(b) Description of Related Art
Yb-doped fiber lasers are used in a variety of applications. As with all lasers, a fiber laser generates coherent light wherein the amplitude, polarization, frequency or wavelength, and phase of the output laser light can be controlled. In general, fiber lasers include an optical pump source, two reflectors comprising the optical cavity of a resonator, and an active region within the cavity. Unlike other lasers, the cavity and active region of a fiber laser are formed from an optical fiber. The fiber generally includes a doped glass core that acts as the laser's active region. In operation, the pump is coupled, via one end of the resonator, to the doped-glass core active region. The ions in the doped core are excited by the pump to generate light that is reflected between the reflectors. At least one of the reflectors of the resonator is partially reflective, thereby allowing a portion of the laser light to escape the cavity as the laser output.
Ytterbium (Yb) doped glass is an attractive core material for making efficient fiber lasers operating in the 970-1150 nm range. Of particular importance is operation at 970-980 nm because this wavelength has good overlap with the 980 nm absorption band of Er-doped fiber amplifiers (EDFA). However, Yb-doped fiber lasers operating at 970-980 nm can be more difficult to implement than Yb-doped lasers operating at 1020-1150 nm. This is because the operation at 1020-1150 nm involves a four-level laser scheme, while at 970-980 nm, the lasing process involves a three-level laser scheme. A four-level scheme, in general, results in a more efficient laser operation than a three-level scheme.
The differences between the three and four-level schemes are best illustrated by reference to the diagram shown in FIG. 1. The diagram illustrates a Yb.sup.3+ energy level structure having a ground manifold, .sup.2 F.sub.7/2 (comprised of energy levels (a)-(b)), and an excited manifold, .sup.2 F.sub.5/2 (comprised of energy levels (e)-(g)). The spectroscopic notations .sup.2 F.sub.7/2 and .sup.2 F.sub.5/2 refer to the corresponding electronic structures of the Yb ion. At room temperature, almost all of the Yb ions reside on the ground level (a) of the lower manifold. The lasing process involves exciting ions in the active doped glass core of the fiber from the ground manifold to the excited manifold using an optical pump source. Pump photons are absorbed by the ions residing on the level (a) of the ground manifold, thereby exciting the ions to the energy levels of the excited manifold that are resonant with the pump radiation, generally to levels above level (e). Some ground level ions may be off-resonance with the pump radiation, and therefore are not excited and remain at level (a). The ions in levels (f) or (g) relax non-radiatively to level (e) followed by stimulated emission to the energy levels of the ground manifold that are resonant with the laser radiation. In a four-level scheme, such levels may only include those above level (a). The radiative transition to those levels is followed by rapid non-radiative decay to the ground level (a). Thus, the four-level transitions may be generally described as level (a) to level (f) (or (g)) to level (e) to level (b) (or (c) or (d)).
Because laser stimulated emission involves only excited levels (b)-(d) of the ground manifold, such radiation is off-resonance with the majority of ions residing on the level of the ground manifold. As a result, such radiation is not absorbed by the un-excited ions and the laser performance is not adversely affected by such absorption. In a three-level scheme, the ion excitation process is similar to that in the four-level scheme, in that it involves pump absorption from level (a) to levels (f) and (g), followed by non-radiative decay to level (e). Some of the ground-level ions may be off-resonance with the pump radiation, and therefore, they are not excited efficiently to the excited manifold. Unlike in the four-level scheme, the stimulated emission at the laser wavelength occurs between levels (e) and (a). Thus, the three-level transitions may be generally described as level (a) to level (f) (or (g)) to level (e) and back to level (a). Because most of the exited ions emit back to the bottom ground manifold level (a), any ions remaining in level (a) can absorb the laser light and thereby negatively affect laser performance.
Thus, there is a need for an improved Yb-doped fiber laser that reduces the inefficiencies originating from the three-level energy scheme that is required for operating such a laser near the absorption band of an Er-doped fiber amplifier (EDFA). In particular, there is a need for a Yb-doped fiber laser that reduces the lasing threshold (i.e., the amount of input power required before lasing begins) and increases slope efficiency (i.e., percentage of input power over the threshold that is converted to lasing power) when operating such a laser near the absorption band of an EDFA.